October shows its sunny disposition

By one measure, San Diego had its warmest October in 21 years. The average temperature for the month at Lindbergh Field was 69.0 degrees. Not since 1987, when the average was 69.5, was October hotter.

But those numbers this year would be higher if there weren't some unusually cold nights tossed in. It got down to 52 on Oct. 12 and 53 on Oct. 13. The average for a day is determined by adding the high and low and dividing by two.

Looking just at the high temperatures, October 2008 was the hottest in 26 years. The average high this year was 78.1. The last time the month topped that number was in 1982, when it was 78.8.

The monthly average has been building over the last five years. Last year, the average high during the month was 73.5. In 2006, it was 71.8; '05: 71.3; and '04: 69.7.

The mercury hit 80 degrees or higher at Lindbergh Field 16 times during October this year. That's more than the months of September, August, July, June and May – combined.

An 80-degree day is actually fairly rare at Lindbergh. It has happened only 36 times this year, and October is the only month when it happened more than five times. The 94 degrees recorded Oct. 8 tied April 27 for the hottest day of the year so far.

That's a bunch of pretty hot numbers. What was going on?

Ivory Small, the science and operations officer at the National Weather Service office in Rancho Bernardo, said there was a stagnant pattern. A powerful ridge of high pressure was parked either right on the West Coast or just offshore most of the month. That pattern isn't too unusual for October, he said, but its strength and persistence was.

“If it's a bigger high than normal, it has a tendency to give you better subsidence (air compressing at the Earth's surface) to keep things warm,” Small said. The pattern also allowed Santa Ana conditions to develop a couple of times, which also elevated the temperatures.

More stats
Inland areas of the county certainly weren't immune to the heat. Reader Carl Carrano compiled these numbers for Ramona last week:

Days over 90 degrees: 15, including a run of eight straight.

Record highs: 5.

Average daily high vs. normal: +5 degrees.

Daily average (high and low) vs. normal: +2.4.

Days with humidity below 10 percent: 15, including eight straight.

Reader Phil Pryde felt the heat, too. But he also noticed some rather huge temperature swings within the same day.

“A 40-degree thermometer is generally considered pretty long, but we have had several 50-degree days,” Pryde said. “As a well-known weather statistics junkie, I've been intrigued by the very long thermometers throughout the county.”

Here are some 50-degree swing days he noted at inland stations last month:

Small said the high pressure that caused the heat prevented an onshore flow most of the month, so the air stayed very dry. Dry air cools off more at night, then warms up quickly during the day.

Raising flags
The heat and low humidity levels kept the weather service very busy during the month. It issued many red-flag warnings for critical fire danger, including over six straight days from Oct. 24 through 29.

But Small said not all red-flag warnings are created equal. The weather service puts out the warnings when one of three criteria are or will be met:

1. When the humidity level in an area is expected to be below 10 percent for 10 consecutive hours.

2. When the forecasters expect sustained winds of 25 mph or greater or gusts of 35 mph or greater, combined with relative humidity of 15 percent or below for six or more hours.

3. When dry lightning is present or likely.

Many of the red-flag warnings issued in October were for the first of the criteria. Humidity levels in some areas of the county backcountry stayed low over many days.

But firefighters are much more concerned when the second of the criteria – the combination of low humidity and strong winds – is met. Dry lightning is a far bigger problem in the Sierra and other mountain ranges of the West. Most thunderstorms in San Diego County's mountains are accompanied by rain.

Visit the Weather Watch blog at Weatherwatch.uniontrib.com . Have a question or comment about the weather? Write to Weather Watch, The San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or send e-mail to
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